Backpack



FIG. 1 is a rear, left, and top perspective view of a backpack showing the new design;

FIG. 2 is a rear view thereof;

FIG. 3 is a front view thereof;

FIG. 4 is a bottom view thereof;

FIG. 5 is a top view thereof;

FIG. 6 is a right view thereof; and,

FIG. 7 is a left view thereof.

The broken lines showing zipper structures depict portions of the backpack that form no part of the claimed design. 

CLAIM The ornamental design for a backpack, as shown and described. 